The law – signed Friday by Gov. Jan Brewer (R) – requires law enforcement to check the residency status of those thought to be in the country illegally. Police unions were divided on the issue and some leading law enforcement agencies petitioned Governor Brewer not to sign the bill – fearing racial profiling and loss of the public's trust.

Police face contradicting missions, critics argue. “This obviously puts police in an impossible situation because it requires them to pursue two goals simultaneously: to enforce the immigration laws; and to enforce the criminal laws, keep the peace, provide assistance, and all the other ordinary tasks of police officers,” says Joel Jacobsen, assistant attorney general, criminal appeals division for New Mexico. “Which goal should they pursue?"

Under the new law, the consequences for victim and perpetrator will not align, says Mr. Jacobsen. "It will frequently not be possible to do both, because the officer will be required to arrest perpetrator and victim both, and the punishment experienced by the victim of a violent crime will frequently be more severe and life-disrupting – deportation – than that experienced by the perpetrator – a night in jail, perhaps.”

Immigrants' rights groups say the new practice virtually ensures that police, untrained in immigration enforcement, will engage in racial profiling as a result of the law. Brewer has said that will not occur. People who look Hispanic or who have Hispanic surnames will be stopped, immigrant rights groups claim, while others will not. President Obama has ordered his staff to keep a close watch on Arizona to see if police step over the line.

Untrained local law enforcement have engaged in racial profiling in the past, one study found. The US Inspector General released a study evaluating Section 287(g) laws – those that in 1995 authorized the federal government to partner with local law enforcement agencies to perform immigration law enforcement functions, says Professor Karthick Ramakrishnan, a political scientist at the University of California, Riverside. “The Inspector General found that, because the local police personnel were not trained, there were instances of racial profiling in the implementation of these laws,” he says.

One, it distracts police from their energies put into other crime and law and order. Two, immigrants are subsequently less likely to report crimes or serve as witnesses if their legal status is going to be questioned.

“The biggest trend in policing in the past two decades has been community policing in which cops walk the local beat and spend much time gaining the trust of the people,” says Ramakrishnan. “This puts that trend entirely in jeopardy – it is a very big deal for them, indeed.”